Two powerful tools for advanced visualization of neurons and neural circuits have recently been developed. The first uses neuroinvasive α Herpesviruses as self-amplifying markers that spread only between snaptically connected neurons. These viruses can be used to selectively label a population of neurons based on genetic and/or connectivity criteria. Further, several known viral strains are defective for either anterograde or retrograde transport form the soma, offering neuroscientists the ability to answer direction-specific questions. The Herpes Simplex Virus type 1 (HSV-1) strain H129 is the only α Herpesvirus which has been observed to move almost exclusively in the anterograde-only direction while HSV-1 MacIntyre moves only in the retrograde direction from the site of infection. The second tool is the Brainbow 1.0L cassette, which expresses the red fluorescent protein tdTomato unless it undergoes Cre-meditated recombination to express either Cyan or Yellow fluorescent protein. The recombination event selectively labels Cre-expressing neurons, and generates a diversity of color that distinguishes individual cells within the target population. Recently neurovirologists have modified
α herpesviruses to express Brainbow. These Brainbow viruses address the weaknesses in each
of the above tools: 1. As informational circuits are not genetically defined, transgenic Brainbow animals could not be used to explore neural circuits; 2. Viral infection with only one labeling protein cannot distinguish between individual neurons in a circuit, particularly at the synaptic level. Here, I have constructed Brainbow- and Cre-encoding HSV-1 unidirectional recombinants.